Conventional Ebert and Czerny-Turner type spectrometers and spectrographs are known in the art and have been common optical instruments for most of this century. These instruments use free-space optical elements--mirrors, reflective gratings, and slits--to disperse incident light into component wavelengths. Recently, there has been some experimental work using waveguides made of plastic or silicon dioxide, rather than air, to transmit the light from one optical element to the next.
Most infrared spectrometers and spectrographs of the prior art use cooled infrared detectors as well as separately-aligned optical elements. Cooling the detectors requires added bulk and heavy equipment, such as closed cycle refrigerators, cryogenic liquid, and thermoelectric coolers with large power supplies. Maintaining optical alignment is also a challenge if the instrument is to be subjected to environmental stresses.
Infrared spectroscopy for chemical analysis has typically used laboratory grade spectrometers, spectrographs, or Fourier Transform Infrared (FTIR) instruments. These instruments can be as small as a cigar box and as large as a table top. They generally consist of entrance and exit slits, reflective mirrors and a reflective diffraction grating made of glass which may or may not be rotated to scan the wavelengths of interest. The FTIR, for example, generally uses a Michelson interferometer with a moving mirror in one interferometer leg and a fixed wavelength reference such as a laser to scan the wavelengths. In addition there is a host of external equipment, including power supplies, scan motors and associated control electronics, and of course a cooled infrared detector. For example, an infrared chemical analysis instrument developed by Foster-Miller uses an FTIR about the size of a breadbox with a liquid nitrogen-cooled mercury cadmium telluride (HgCdTe) detector.
There are a few spectrometers, in the prior art, that attempt to remedy the above-described problems of size, weight, complexity and power consumption. Zeiss manufactures a Monolithic Miniature-Spectrometer, or MMS 1, and distributes the MMS 1 through Hellma International, Inc., of Forest Hills, N.Y. The MMS 1 uses a conventional silicon photodiode array and a cylinder of glass with an integral imaging grating. However, its operation is limited by the photodiode and the spectral limitations of the glass relative to ultraviolet (UV) and visible wavelengths. The detector is also positioned away from the waveguide, presenting certain optical alignment difficulties, particularly under environmental stresses.
Another instrument has been demonstrated by researchers at Kernforschungszentrum Karlsruhe GmbH (U.S. representative American Laubscher Corp., Farmingdale, N.Y.). It uses a waveguide and grating fabricated in a multi-layer photoresist of polymethyl methacrylate (PMMA) to couple the output of an optical fiber to a linear detector array or array of fibers. This instrument is designed for demultiplexing applications. As above, because of the choice of materials, operation of this device is limited to visible and near-IR wavelengths of 600 nm to 1300 nm. Specifically, fabrication of the waveguide and grating is done by a process called LIGA (which stands for the German words for Lithography, Galvanoformung and Abformung) which uses a three-layer photoresist and deep-etch X-ray lithography.
Yet another instrument has been developed by researchers at Oak Ridge National Laboratory. It is a microspectrometer based on a modified Czerny-Turner configuration and machined from a block of PMMA also known as Acrylic.TM. material or Plexiglas.TM. material. It has a bandwidth of about 1 .mu.m centered at 980 nm and uses an externally mounted linear photodiode array.
None of these prior art spectrometers permit fully monolithic operation and spectral sensitivity from 1.1 .mu.m-12 .mu.m, or further, and with uncooled detectors.
It is, accordingly, an object of the invention to provide apparatus that solves or reduces the above-described problems in the prior art.
Another object of the invention is to provide a hand-held, monolithic, rugged IR spectrometer.
A further object of the invention is to provide methods of manufacturing uncooled IR spectrographs.
Yet another object of the invention is to provide a process of isolating chemical species over a large infrared band with a monolithically-constructed IR spectrometer.
These and other objects will become apparent in the description which follows.